Here is the simple arrangement of the Sim300 gsm modem with 8051projects board.
AT Commands to initialize the gsm modem
AT => Its just to wake the modem.
ATE0 => Echo off
AT+CMGF=1 => Enable text mode to send sms
AT+CNMI=1,1,0,0,0 => Disable new message indication OFF
AT Commands to Send/Receive SMS
AT+CMGF=1 =>Make SIM300 to text mode
AT+CMGR=1 =>To read the first message
AT+CMGS="mobile number" =>To send SMS
AT+CMGD=1 => To delete first message
AT+CMGD=1,4 =>To delete all messages
AT Commands to make/receive call
ATDmobilenumber => To make a call
ATH => To Hang/disconnect a call
ATA =>To receive a call
ATE0 => Enable caller ID
Send and Receive SMS through GSM modem 1.0
Bascom 8051 code to send and receive sms using SIM300 gsm modem.